Unexpected failure of a motor in an industrial or military environment can result in costly downtime, increased safety hazards and possible catastrophic failure of other components or processes. Over the years, a variety of periodic test procedures have been developed to allow the prediction of incipient failures in motors. Most have functioned in a less than satisfactory manner.
Insulation evaluation techniques, such as "Megger" testing, are popular and have been performed for many years. Those tests involve a direct-current method and are convenient to use. A megohmmeter is typically used although high-potential testers may also be used.
Three types of DC tests are typically employed: the time resistance test, dielectric absorption ratio and the step voltage test. Such DC tests are only capable of revealing some incipient problems, and their use requires that the electric machinery be taken off line and tested in a non-operational mode. Furthermore, the results of DC tests are relative and therefore, their evaluation is based on trends over time, rather than on a specific value of one reading.
Other tests, including partial-discharge tests, detect charge transfer in insulation voids under an applied AC voltage. Power-factor tests measure dielectric power factor and use it as an evaluation parameter. Surge tests utilize the natural response of the windings to an impressed transient voltage to detect winding or dielectric defects; typically windings are examined in pairs, with the assumption that any differences between the responses of the two windings are indicative of defects. The test apparatus for performing such tests is typically expensive and the test procedures are difficult to employ, requiring skilled personnel and "near-laboratory-like" conditions to perform the tests successfully. Their use also requires that the electric machinery be taken off line and tested in a non-operation mode. Moreover, surge testing can be destructive to the motor under test.
There is a need for a monitoring system that can detect all modes of incipient failures in stator windings of electric machines and other common failure modes in the rotor. The method should pose no risk of damage to insulation systems during its use and should be able to be used outside of a test facility and while the electric machine is in operation. It should further be immune to power system variations and transients which could mask an incipient failure condition.
Recently, the inventors hereof found that early detection of incipient failures in electrical motors can be achieved by the monitoring of changes in negative sequence current and impedance of a multi-phase motor (see: "Alternative For Assessing the Electrical Integrity of Induction Motors", Kohler et al., Conference record of the 1989 IEEE Industry Application Society Annual Meeting part II, Oct. 1-5, 1989.).
Before discussing the findings of Kohler et al., it is worthwhile to review the method for dealing with unbalanced polyphase circuits, termed "method of symmetrical components" that was first published by C. L. Fortescue in "Method of Symmetrical coordinates Applied to the Solution of Polyphase Networks", Transactions of the AIEE Volume 37, pages 1027-1140, 1918.
The symmetrical component method provides a convenient technique for simplifying calculations relating to unbalanced power systems. Most power system calculations are performed on a per-phase basis, with an assumption that the system is perfectly balanced. As shown in FIG. 1, a balanced three phase system means that the three voltage phasors V.sub.A, V.sub.C, V.sub.B each have an equal magnitude and are phase displaced by 120.degree.. The same relationship must also exist for the three current phasors. If however, phasors exist such as shown in FIG. 2, where the magnitude of the voltage or current phasors are unequal, and/or the angles therebetween are different, then the application of classical circuit-analysis techniques becomes cumbersome.
Fortescue found that an unbalanced system of n phasors could be resolved into n systems of balanced phasors (called the "symmetrical components") of the original phasors. The n phasors of each set of components are balanced, that is, equal in magnitude and the angles between adjacent phasors of the set are also equal. Thus, an unbalanced system can be analyzed as a group of balanced systems, thereby simplifying the analysis.
The symmetrical components for an unbalanced three-phase system of phasors, as defined by Fortescue, are shown in FIGS. 3, 4, and 5. In FIG. 3, positive sequence components are shown and consist of three phasors that are equal in magnitude, are displaced from each other by 120.degree., and have the same phase sequence as the original phasors. In FIG. 4, negative-sequence components are shown and consist of three phasors that are equal in magnitude, are displaced from each other by 120.degree. and have a phase sequence that is opposite to that of the positive sequence phasors. In FIG. 5, zero-sequence components are shown and consist of three phasors that are equal in magnitude and exhibit a zero phase displacement from each other. Given that the phasors shown in FIGS. 3, 4, and 5 are from an unbalanced three phase supply, their respective vector additions will result in the phasor values shown in FIG. 6, which represents the original phasors of an unbalanced three phase system. An important premise of the theory of symmetrical components is that the individual positive, negative and zero sequence components are independent and do not react, one upon the other for a symmetrical machine. Using the theory of symmetrical components analysis, the presence of only positive-sequence voltages and currents indicates a perfectly balanced power system, serving balanced loads. The presence of negative-sequence values indicate some level of system imbalance (faults, single phase loads, etc.). The presence of a zero-sequence component represents a fault involving ground.
Kohler et al, in the above noted IEEE paper, found that negative sequence components (i.e., current, voltage and impedance) bore a direct relationship to incipient failures in electric motors. First, a motor's negative sequence impedance is relatively insensitive to speed changes and other non-motor anomalies. Kohler et al demonstrated that high-impedance leakage paths within motor windings could be identified by measurements of negative-sequence currents. Such negative sequence currents were found to be directly proportional to leakage currents at constant voltages. Furthermore, for such constant leakage currents, the level of negative sequence current increased with increasing potential--thus reflecting an increase in deterioration severity. Kohler et al further found that the level of predictive ability achieved by monitoring of negative-sequence currents could be adversely affected by supply line variations, but that the calculation and use of the negative sequence current to an effective-negative-sequence impedance negated such variations. Thus, by monitoring of negative-sequence impedance, incipient motor failure modes could be an effective metric for monitoring the state of an in-service motor.
While the monitoring of changes in negative sequence impedance has been found to be an effective means of monitoring incipient failure modes, certain failure modes exhibit relatively small changes in the negative sequence impedance. Such small changes may be masked by detection system margins and other variables.
Accordingly, it is an object of this invention to provide an improved method for monitoring electric motors for incipient failure modes.
It is another object of this invention to provide improved negative sequence component monitoring which enables earlier and more precise identification of incipient failure modes.